But if it belongs to the BRF why we haven't seen it before on the queen or Mathilde? And I thought that we've already seen what their vaults have and that there could be no surprises there. It can be very well a loan from someone.

Maybe it is a new creation? Or something in the vault that no one has worn before?
There are plenty of tiaras and jewels hanging around in the British Vaults that haven't been worn for ages, we would probably think them as new or borrowed.
I don't see why she would borrow on, when she could have worn one from the vaults.

I'm pretty sure that the Belgium RF doesn't have any unused tiaras lying around. It was often told on TRF how Princess Lilian de Rethy sold out on the royal jewellery after her husband death. The Queens Fabiola and Paola had to make do with very few pieces which they repeated again and again. You can't compare them to the British royals who are and were much richer, and never seriously sold jewellery.
I also doubt that Prince Laurent bought the tiara for his wife. He seems to enjoy fast cars more than royal dignity, and also he isn't exactly rolling in money. And wasn't it just last year that Laurent and Claire enrolled their kids in a French school because it was less expensive than the previous school? Why would he buy another tiara when even his sister in law, the crown princess, has to get along with only one tiara?
Sorry if I sound very pessimistic, but I do believe that the tiara was just a loan.

I do not want - and I am not able - to discuss the financial situation of the Belgium RF, however I would expect that they are not so badly situated that there is no chance for one or the other new tiara within 20 years or so....given that historical pieces (don't know about new ones) seem to be not that expensive....(only know from Christies etc. auctions).
I would assume it is just something which does not matter to them...

Just a very humble queston: why should PC BORROW a tiara?
I can imaging borrowing from the family - but from friends? Would a princess from reigning royal family with her own tiara on hand do something like that? This will remain a mystery.